The present invention relates to an image transmission method and an image transmission system and, more particularly, to an image transmission method and an image transmission system for transmitting an image from an image transmitting apparatus to a predetermined image receiving apparatus through a transmission line in accordance with an instruction for transmitting an image.
A monitoring system is known for monitoring the situation of each point by a plurality of monitors provided at a monitoring center by appropriately displaying an image which is caught by a monitoring camera provided at each point and transmitted to the monitoring center through a network. For example, a monitoring system for monitoring a traffic volume in a highway network analyzes the image transmitted from each point so as to grasp the state of a traffic jam, and supplies traffic information. Another monitoring system analyzes the image transmitted from a monitoring camera set at each point which is in danger of landslide so as to grasp the landslide state, and gives an alarm.
FIG. 11 shows the structure of an example of conventional monitoring systems having a digital exchange 3, and transfer devices 41 to 4n between image transmitting apparatuses 11 to 1n and image receiving apparatuses 21to 2m. Digital transmission lines 51 to 5n connect each of the image transmitting apparatuses 11 to 1n to the corresponding transfer devices 41 to 4n. All of the image transmitting apparatuses 11 to 1n have the same structure, which is composed of a monitoring camera 1a, an image encoder 1b for encoding the image caught by the monitoring camera 1a and outputting the code, and a transmitter 1c for transmitting the coded image data to the digital exchange 3. The digital exchange 3 has a switching function, and inputs the image data transmitted from a predetermined one of the image transmitting apparatuses 11 to 1n to a predetermined one of the image receiving apparatuses 21 to 2m under an instruction from a control unit 6. The control unit 6 inputs an instruction for switching control to the digital exchange 3 on the basis of the receiving apparatus chosen by the operation of an operator. All of the image receiving apparatuses 21 to 2m have the same structure, which is composed of an image receiver 2a and a monitor 2b. The image receiver 2a decodes the coded image data input from the digital exchange 3 to the original image data, converts the digital image data to analog image data, and inputs the analog image data to the monitor 2b. The monitor 2b displays the image caught by the predetermined monitoring camera 1a. 
The image receiving apparatuses 21 to 2m, the digital exchange 3, transfer devices 41 to 4n, and the control unit 6 are provided together at a first point, which serves as a center. In this case, if it is necessary to watch a received image by an image receiving apparatus 7 which is provided at another point, a transfer device 8 is provided on the output side of the digital exchange 3, a transmission line 9 is provided between the transfer device 8 and the image receiving apparatus 7, and the image receiving apparatus 7 is composed of a transfer device 7a, an image receiver 7b and a monitor 7c. 
FIG. 12 shows the structure of another monitoring system as an example of a system having an analog image exchange 13, transfer devices 141 to 14n, and image receiving apparatuses 151 to 15n between image transmitting apparatuses 111 to 11n and monitors 121 to 12m. Digital transmission lines 161 to 16n connect each of the image transmitting apparatuses 111 to 11n and monitors 121 to 12m. Digital transmission lines 161 to 16n connect each of the image transmitting apparatuses 111 to 11n to the corresponding transfer devices 141 to 14n. All of the image transmitting apparatuses 111 to 11n have the same structure, which is composed of a monitoring camera 11a, an image encoder 11b for encoding the image caught by the monitoring camera 11a and outputting the code, and a transmitter 11c for transmitting the coded image data to the analog image exchange 13. The image receiving apparatuses 151 to 15n decode the coded image data input from the corresponding transfer devices 141 to 14n to the original image data, convert the digital image data to analog image data and input the analog image data to the analog image exchange 13. The analog image exchange 13 has a switching function, and inputs the image signal transmitted from a predetermined one of the image transmitting apparatuses 111 to 11n to a predetermined one of the monitors 121 to 12m under an instruction from a control unit 17. Each of the monitors 121 to 12m displays the image caught by the corresponding monitoring camera 11a. The control unit 17 inputs an instruction for switching control to the analog image exchange 13 on the basis of the operation of a control panel (not shown) by an operator.
The monitors 121 to 12m, the analog image exchange 13, the transfer devices 141 to 14n, the image receiving apparatuses 151 to 15n, and the control unit 17 are provided together at a first point, which serves as a center. In this case, if it is necessary to watch the images transmitted from the image transmitting apparatuses 111 to 11n by an image receiving apparatus 18 which is provided at another point, an image encoder 19 and a transfer device 20 are provided on the output side of the analog image exchange 13, a transmission line 21 is provided between the transfer device 20 and the image receiving apparatus 18, and the image receiving apparatus 18 is composed of a transfer device 18a, an image receiver 18b and a monitor 18c. 
FIG. 13 shows the structure of a monitoring system proposed by the inventor. In this structure, n transmission lines 161 to 16n in the monitoring system shown in FIG. 12 are substituted by one annular transmission line 21. The same reference numerals are provided for the elements which are the same as those shown in FIG. 12.
A plurality of image transmitting apparatuses 111 to 11n are connected to the image transmitting side of the annular transmission line 21, while a plurality of image receiving apparatuses 151 to 15n are connected to the image receiving side of the annular transmission line 21. A plurality of logical channels C1 to Cn are set on the annular transmission line 21. The logical channels C1 to Cn are fixedly allotted to the image transmitting apparatuses 111 to 11n,respectively, and they are also fixedly allotted to the image receiving apparatuses 151 to 15n, respectively.
In this monitoring system, the image transmitting apparatus 111 (i=1, 2, . . . n) transmits an image to the image receiving apparatus 15i through the logical channel Ci, and the image receiving apparatus 15i decodes the received coded image data to the original image data, converts the digital image data to analog image data and, inputs the analog image data to the analog image exchange 13. The analog image exchange 13 has a switching function, and inputs the image signal input from the image receiving apparatus 15i to a predetermined one of the monitors 121 to 12m under an instruction from the control unit 17. Each of the monitors 121 to 12m displays the image caught by a monitoring camera.
In the prior art shown in FIG. 11, it is necessary to provide not only a digital image exchange, but also one transfer device in correspondence to one image transmitting apparatus. Consequently, the monitoring system has a large and complicated structure. The place where the monitoring system is disposed therefore requires a large area, and the cost of the monitoring system as a whole becomes disadvantageously high. In addition, in order to see the received images at another point, it is necessary to transmit the output of the digital exchange through a transfer device, which only aggravates the above-described problems.
In the prior art shown in FIG. 12 or the system shown in FIG. 13, it is necessary to provide not only an analog image exchange, but also one transfer device and one image receiver in correspondence to one image transmitting apparatus. Consequently, the monitoring system has a larger and more complicated structure than the monitoring system shown in FIG. 11. The place where the monitoring system is disposed requires a larger area, and the cost of the monitoring system as a whole becomes disadvantageously higher. In addition, in order to see the received images at another point, since it is necessary to reconvert the output (analog image) of the digital exchange by an image encoder to digital data and transmit the data through a transfer device, the deterioration of the picture quality is inevitable.
Accordingly, it is an object of the present invention to eliminate the above-described problems in the related art and to provide a monitoring system which is dispensed with an image exchange.
It is another object of the present invention to prevent a monitoring system from an increase in size and cost.
It is still another object of the present invention to provide a monitoring system which is capable of transmitting a monitored image from an image transmitting apparatus to any image receiving apparatus and which is also capable of simultaneously transmitting a monitored image to a plurality of image receiving apparatuses.
To achieve these objects, in a monitoring system provided in a first aspect of the present invention, (1) a plurality of image transmitting apparatuses are connected to the image transmitting side of a transmission line, and a plurality of image receiving apparatuses are connected to the image receiving side of the transmission line, (2) a plurality of logical channels are set on the transmission line, (3) a predetermined logical channel is fixedly allotted to each image receiving apparatus, (4) each image transmitting apparatus has a function of transmitting an image to an image receiving apparatus through any logical channel, and (5) each image transmitting apparatus transmits an image to a designated image receiving apparatus through a predetermined logical channel in accordance with an instruction for image transmission. In addition, one image transmitting apparatus simultaneously transmits an image to a plurality of image receiving apparatuses through a plurality of logical channels in accordance with an instruction for image transmission. According to the present invention having the above-described structure, an image exchange is obviated, and image receiving apparatuses provided in correspondence to monitors suffice, resulting in that it is possible to prevent a monitoring system from an increase in size and cost.
In a modification of the above-described structure, an encoder of each image transmitting apparatus is provided with an intra-frame coding system and an inter-frame coding system, and image data encoded by the intra-frame coding system are transmitted for a predetermined period of time at the start of image transmission. In this manner, it is possible to display an image on the monitor of an image receiving apparatus instantly without a temporal delay.
In another modification of the above-described structure, a first annular transmission line for transmitting image data counterclockwise, and a second annular transmission line for transmitting image data clockwise are provided as the transmission line, and the image transmitting apparatuses and the image receiving apparatuses are connected to each of the annular transmission lines. An image transmitting apparatus transmits an image through the same logical channel on both annular transmission lines. An image receiving apparatus receives the image from one of the transmission lines when the transmission line is normal, while receiving the image from the other transmission line when it is impossible to receive a normal image from the one transmission line. In this manner, each image receiving apparatus is capable of receiving and displaying an image continuously, even if a trouble is caused on either or both of the transmission lines.
An image transmitting apparatus provided in a second aspect of the present invention comprises (1) a line interface portion for receiving a frame signal composed of multiplexed image data of the logical channels from a transmission line, and transmitting a frame signal composed of image data to be transmitted from the image transmitting apparatus and the received multiplexed image data to the transmission line, (2) a separator for separating the frame signal transmitted from the transmission line into image data for the respective logical channels, (3) an A/D converter for converting analog image data to digital image data to be transmitted, (4) a memory for storing the digital image data converted by the A/D converter, (5) an image encoder for encoding the digital image data, (6) a multiplexer for being input the coded image data and the separated image data which is separated by the separator, multiplexing these image data by inserting the coded image data into a designated logical channel, and outputting the multiplexed image data to the line interface portion, and (7) a control unit for controlling the multiplexer so as to insert the coded image data into a predetermined logical channel in accordance with an instruction for image transmission. According to the image transmitting apparatus having the above-described structure, an image exchange is obviated.
An image receiving apparatus provided in a third aspect of the present invention comprises (1) a line interface portion for receiving a frame signal composed of multiplexed image data of a plurality of logical channels from a transmission line, separating and outputting the image data of the logical channel which is allotted to the image receiving apparatus, (2) a synchronization detector for detecting synchronization and abnormal synchronization on the basis of the synchronizing data which is added to image data, (3) a decoder for decoding coded image data, (4) a memory for storing the image data for at least one picture which is output from the decoder, (5) a control unit for freezing or releasing freeze depending upon whether the signal output from the synchronization detector is an abnormal synchronization signal or a normal synchronization signal, (6) a selector for selecting the image data output from the decoder when synchronization is detected, while selecting the image data read from the memory when synchronization is abnormal, and (7) a D/A converter for converting the digital image data output from the selector to analog image data. According to this image receiving apparatus, it is possible to receive and display the image transmitted from a predetermined image transmitting apparatus. In addition, even if abnormal synchronization is caused, since the preceding image is displayed (which is called xe2x80x98freezexe2x80x99), there is no display distortion.